My network configuration and port settings have not changed, and my server used to show up just fine. I can only guess that some automated algorithm blacklisted my IP, since I've been starting up and shutting down my server several times over a relatively short period (maybe around 5-10 minutes between each startup and shutdown), while performing test runs to ensure that I have the server settings (map rotations and such) configured correctly, before keeping the server up long-term for public use.